Astrophysicists led by the UNIGE and the NCCR PlanetS have investigated the
past of Venus to find out whether Earth’s sister planet once had oceans.
The planet Venus can be seen as the Earth’s evil twin. At first sight, it is
of comparable mass and size as our home planet, similarly consists mostly of
rocky material, holds some water and has an atmosphere. Yet, a closer look
reveals striking differences between them: Venus’ thick CO2 atmosphere,
extreme surface temperature and pressure, and sulfuric acid clouds are
indeed a stark contrast to the conditions needed for life on Earth. This
may, however, have not always been the case.
Previous studies have suggested that Venus may have been a much more
hospitable place in the past, with its own liquid water oceans. A team of
astrophysicists led by the University of Geneva (UNIGE) and the National
Centre of Competence in Research (NCCR) PlanetS, Switzerland, investigated
whether our planet’s twin did indeed have milder periods. The results,
published in the journal Nature, suggest that this is not the case.
Venus has recently become an important research topic for astrophysicists.
ESA and NASA have decided this year to send no less than three space
exploration missions over the next decade to the second closest planet to
the Sun. One of the key questions these missions aim to answer is whether or
not Venus ever hosted early oceans. Astrophysicists led by Martin Turbet,
researcher at the Department of Astronomy of the Faculty of Science of the
UNIGE and member of the NCCR PlanetS, have tried to answer this question
with the tools available on Earth.
“We simulated the climate of the Earth and Venus at the very beginning of
their evolution, more than four billion years ago, when the surface of the
planets was still molten,” explains Martin Turbet. “The associated high
temperatures meant that any water would have been present in the form of
steam, as in a gigantic pressure cooker.”
Using sophisticated three-dimensional models of the atmosphere, similar to
those scientists use to simulate the Earth’s current climate and future
evolution, the team studied how the atmospheres of the two planets would
evolve over time and whether oceans could form in the process.
“Thanks to our simulations, we were able to show that the climatic
conditions did not allow water vapor to condense in the atmosphere of
Venus,” says Martin Turbet. This means that the temperatures never got low
enough for the water in its atmosphere to form raindrops that could fall on
its surface. Instead, water remained as a gas in the atmosphere, and oceans
never formed. “One of the main reasons for this is the clouds that form
preferentially on the night side of the planet. These clouds cause a very
powerful greenhouse effect that prevented Venus from cooling as quickly as
previously thought,” continues the Geneva researcher.
Small differences with serious consequences
Surprisingly, the astrophysicists’ simulations also reveal that the Earth
could easily have suffered the same fate as Venus. If the Earth had been
just a little closer to the Sun, or if the Sun had shone as brightly in its
‘youth’ as it does nowadays, our home planet would look very different
today. It is likely the relatively weak radiation of the young Sun that
allowed the Earth to cool down enough to condense the water that forms our
oceans. For Emeline Bolmont, professor at UNIGE, member of PlaneS and
co-author of the study, “this is a complete reversal in the way we look at
what has long been called the ‘Faint Young Sun paradox’. It has always been
considered as a major obstacle to the appearance of life on Earth!” The
argument was that if the Sun’s radiation was much weaker than today, it
would have turned the Earth into a ball of ice hostile to life. “But it
turns out that for the young, very hot Earth, this weak Sun may have in fact
been an unhoped-for opportunity,” continues the researcher.
“Our results are based on theoretical models and are an important
building-block in answering the question of the history of Venus,” says
study co-author David Ehrenreich, professor in the Department of Astronomy
at UNIGE and member of the NCCR PlanetS. “But we will not be able to rule on
the matter definitively on our computers. The observations of the three
future Venusian space missions will be essential to confirm – or refute –
our work.” These prospects delight Emeline Bolmont, for whom “these
fascinating questions can be addressed by the new Centre for Life in the
Universe, which has just been set up within the UNIGE’s Faculty of Science.”
Reference:
Day–night cloud asymmetry prevents early oceans on Venus but not on Earth by
Martin Turbet, Emeline Bolmont, Guillaume Chaverot, David Ehrenreich, Jérémy
Leconte and Emmanuel Marcq, 13 October 2021, Nature.
DOI: 10.1038/s41586-021-03873-w
Tags:
Space & Astrophysics